Somatic characteristic, morphological asymmetry and postural stability of youth male canoeists compared to control. A cross-sectional study

Objective To determine the relationships between selected morphological characteristics and the level of body composition asymmetry with postural stability among canoeists and a control group. Methods The sample consisted of 43 males (21 canoeists 21.8±3.29 years and 22 university students 21.7±1.32 years). Measurements included body height and weight. Segmental body composition analysis was assessed using the bioelectrical impedance by determining: fat mass (FM, kg, %), fat-free mass (FFM, kg) and predicted muscle mass (PMM, kg). Postural stability was tested using the BIODEX Balance System. Anterior–posterior stability index (APSI), medial–lateral stability index (MLSI) and overall stability index (OSI) were calculated. Results Our findings suggest that the canoeists had statistically lower level of fatty tissue compared to controls. There was a statistically important difference between groups in lower limb FM (% and kg). In both groups morphological asymmetry was observed, but in most cases—in athletes. Asymmetries between right and left arms appeared in all parameters, while asymmetries between right and left legs were noted in all parameters except FM (kg). There were relationships between stature and body weight with postural stability in canoeists. Canoeists demonstrated better balance than controls, particularly in the APSI. For all stability indices, significant differences were observed between right and left legs across all participants. Conclusions Athletes, with larger asymmetries or poorer balance, require more attention to improve performance and reduce the risk of overload injury. Future studies needed to develop sport-specific level of morphofunctional asymmetry which would be optimal for sport results and health as well.


Results
Our findings suggest that the canoeists had statistically lower level of fatty tissue compared to controls. There was a statistically important difference between groups in lower limb FM (% and kg). In both groups morphological asymmetry was observed, but in most cases-in athletes. Asymmetries between right and left arms appeared in all parameters, while asymmetries between right and left legs were noted in all parameters except FM (kg). There were relationships between stature and body weight with postural stability in canoeists. Canoeists demonstrated better balance than controls, particularly in the APSI. For all stability indices, significant differences were observed between right and left legs across all participants.

Introduction
There are many factors that affect athletic performance like genetic potential, occupation, physical environment, daily nutrition, quality of sleep, training, injury coach-athlete relationship etc. [1]. Also, body structure and body composition (fat, muscle mass, total body water) are important determinants of performance in many sports [2]. Paddle sports have two subdisciplines: canoeing and kayaking. There are a few differences between them. In a kayak, the paddler is seated and uses a double-bladed paddle pulling the blade through the water on alternate sides to move forward. In a canoe, on the other hand, the paddler kneels and uses a single-bladed paddle to propel the boat forward [3]. This has its consequences in the form of specific movement patterns. Canoeists athletes perform more asymmetrical moves. Canoeing has numerous requirements [4], including specific athlete anthropometric parameters [5,6], which carry great importance. Hamano [7] reported on the significance of monitoring canoe athletes' somatic characteristics and body composition during the process of planning training programmes and identifying athletes with a predisposition for this sport. Compared to kayak athletes, canoe athletes have smaller values for body height, sitting height, body weight, arm span, upper limb length and multiple body part circumferences [8,9]. Ackland [9] showed that elite canoe sprinters have a more developed upper body, smaller buttocks and higher lean body mass values compared to beginner athletes and non-athletes. It has also been stated that appropriate shoulder and calf circumference sizes may be important in this sport [7]. Canoe athletes have been found to have 12%-18% body fat [10]. In athletes, there is a phenomenon of somatic adaptation to the specificity of a given discipline and its associated training loads, as athletes with different body structures can react to training loads in different ways. These discipline-specific loads are often reflected in the somatic parameters of athletes and are particularly noticeable in the case of unilateral sports. Many studies have shown that asymmetric sports disciplines lead to side-to-side differences in a variety of parameters [11][12][13]. One of them is canoeing. In classic canoeing-flatwater, competitors paddle in kayaks or in canoes. Whereas paddling in a kayak requires symmetric movements, paddlers in a canoe perform asymmetrical moves [14]. It is characterised by numerous one-sided activities, including the paddling technique (one-turn paddle) and the characteristic position of the athlete in the boat (half-kneeling in a lunge position), which can contribute to an asymmetric distribution of strength in certain muscle groups [15] and unbalanced morphological characteristics (muscle and fat). During paddling, the asymmetric tension of the lower limb musculature can cause asymmetries in body structure, particularly asymmetric muscle hypertrophy [16]. It can also lead to unbalanced forces on the boat.
Extreme specialisation (excessive practice of specific techniques) in sports such as fencing or canoeing can induce strong preferences for one side of the body and create asymmetric balance patterns [17,18]. Also, in team games like hockey or handball asymmetrical positions are often assumed what could affects body posture and its stability [17,19]. It is worth to know that some asymmetry level is also observed in population studies [20,21].
Balance is fundamental motor skill providing the basis for the most movement task [22], particularly in its protective mechanism against injuries and is especially valid in the case of repetitive training over many years due to excessive practice of specific techniques. It can be deepened over time as a result of many years of unilateral training in sports such as canoeing, particularly as symmetrical training is generally not a focus in these sports [23,24]. In many sports, though particularly in unilateral sports, athletes inevitably develop some degree of asymmetry; while, at a certain level, this is necessary to optimise performance, in excess, asymmetry can be unhealthy and inhibit performance. Balance is understood to be the ability to protect a given posture with minimal movement during fluctuations that are caused by static or dynamic conditions [25]. Static balance is the ability to maintain body position with minimal deviation while in a fixed position and on a stable surface [26], while dynamic balance is the ability to maintain body position while the body is moving or on an unstable surface [26,27]. From a biomechanical point of view, balance is a state of the body in which the sum of the forces acting on it is equal to zero; the stabilising internal forces (motor effects of the nervous system) are equal to the destabilising external forces [28]. In canoeing, an incorrect movement or loss of balance can cause less effective work or even a fall overboard; balance is thus a critical component of this discipline [29].
Canoeing is one of the disciplines that involves the asymmetrical performance of motor activities. It also requires an above-average level of balance and nervous system efficiency due to the performance of starting activities on an unstable surface such as water. Estimating the level of asymmetry provides an opportunity to counteract unilateral overloading of the body, which can lead to injury in extreme cases. It was concluded in the literature that asymmetry of athletes is one of the main causes of musculoskeletal diseases and sports injuries [30]. This asymmetry could be caused f.e. by abnormality of the spine [31]. Moreover, asymmetry of bilateral loading can contribute to the increase of unilateral limb damage and injuries [32]. In asymmetry sport disciplines uneven development of muscle mass may cause disorders of posture, leading to negative changes in the skeletal system [33]. It was also emphasizing that lower limbs muscle strength asymmetry is associated with a high risk of injury [34,35].
The true is however that there are inconsistent findings about eventual association between bilateral lower limb asymmetries and injury occurrence, even less is known about upper limb asymmetries (which would be important, especially in highly asymmetric sports such as canoeing) and injury occurrence. That is why the reduction of injury occurrence may be achieved through the identification of risk factors, which are specific for certain sport, also connected with morpho-functional body asymmetries what could be helpful for coaches.
In the context of the above issues, the main purpose of this study was to determine the relationships between selected morphological characteristics and the level of body composition asymmetry with postural stability among canoeists and a control group.

Material and methods
The study group consisted of 21 male canoeists aged 21.8±2.29 years from the Wielkopolska region of Poland; all participants had 1.5 to 5 years of practice national or international level. The studies were carried out taking into account the training periodization, at the beginning of the preparation period. The control group was made up of 22 university students aged 21.7 ±1.32 years who were engaged in recreational sports activities but were not part of any competitive sports training.
All participants received clear information regarding the study, including the risks and benefits of participation, and provided written informed consent. The study was conducted according to the latest version of the Declaration of Helsinki, and the local ethics board of Poznan University of Medical Sciences approved the study protocol (approval number: 172/16).

Study protocol
Before the measurements all participants completed a questionnaire. Prior to beginning the research, all participants declared their hand preference (which hand they write with) [36].
The next step was anthropometry protocol as well as body composition measurement. The last stage of research was postural stability assessment.

Anthropometric measurements and body composition analysis
Anthropometric measurements were performed by an experienced researcher following the recommendations of Martin and Saller [37]. Body height was measured by stadiometer (GPM, Switzerland), to the nearest 1 mm.
Body weight and segmental body composition were determined using the Tanita analyser (BIA; Tanita MC-780MA analyser, Japan). Following parameters were assessed: fat mass by percentage and in kilograms (FM% and FMkg), fat-free mass in kilograms (FFMkg) and muscle mass in kilograms (PMMkg) for right and left upper and lower limbs, based on the directions and procedures of the manufacturer [38]. All subjects were asked to refrain from drinking or eating, empty their bladders, wear light clothing and remove metal objects before the examination [39]. During the test, participants stood erect with their bare feet on the contact electrodes while holding the electrodes of the BIA unit in their hands. All assessments were completed in a single visit scheduled in the morning.
For each subject, all measurements (anthropometric and BIA measurements) were taken on the same day in an indoor environment, where the temperature was maintained at a constant 22˚C.

Postural stability measurement
All participants underwent postural stability testing on the BIODEX Balance System SD (BBS; Biodex, Shirley, NY, USA) by the same well-trained researcher. A stabilographic platform was employed, which enables the measurement of changes in the forces acting on the ground in the anterior-posterior or lateral directions that affect the point of application of the resultant force, which is referred to as the centre of pressure (COP) [28]. The platform is an instrument designed to measure and train postural stability on fixed or unstable surfaces. The BBS system consists of a circular platform that can move freely and simultaneously in the front-rear and centre-lateral axes and has a display that provides real-time feedback on the subject's posture.
Participants stood on the BBS platform in double-leg and single-leg (right and left leg) positions, keeping their eyes open and looking at the screen throughout all measurement trials. All trials were conducted in athletic shoes, and foot position was recorded using coordinates on a platform grid and entered into the instrument computer. During the measurements, the subjects held their arms freely at their sides and were verbally instructed to stand as still as possible.
A protocol to determine postural stability (Postural Stability Test) with a standard software configuration was implemented on the platform. The protocol consisted of 3 trials of 20 seconds each with a 10-second interval between trials during which the platform was stationary. The interval between double-leg and right and left single-leg tests was 1 minute. The result was the average value of the tree values [40].
The stability indices anterior-posterior stability index (APSI), medial-lateral stability index (MLSI) and overall stability index (OSI; a combination of the APSI and MLSI that is sensitive to changes in both directions) showing the variance of the movement of the center of gravity expressed in degrees, were used for statistical analysis. All stability indices were determined in the following manner [41]: A score of zero represents complete stability, and the larger the deviation from zero, the larger the instability and postural sway [42], indicating difficulty maintaining balance. For each type of deviation, the standard variation of the individual indicators was also automatically provided by the device.
The results of the measurements were stored in the BBS stabilometer, and after the completion of the testing, the mean values of the balance indices were calculated. The athletes and controls were verbally familiarised with the assessment protocol and a 10-minute warm-up was performed by all participants prior to the initiation of the testing protocols.

Data analysis
All parameters were reported as mean values and standard deviation. The normal distribution of the data was tested by the Shapiro-Wilk test and homoscedasticity was assessed by Levene's test.
Descriptive characteristics were compared to explore possible differences between canoeists and controls in height, weight and stability indices. For parameters with normal distribution and homogeneity, Student's t-test for independent sample (t) was used, and for non-normally distributed variables, the Mann-Whitney U (Z) test was used. Multivariate analysis of variance (multifactorial ANOVA leg x group) (F) was used to examine differences in the simultaneous effect of leg (right-left) and group (canoeists-controls) as well as leg-group interactions. Tukey's post hoc procedure was used to identify specific statistically important differences in effects and interactions. The significance of differences in mean values between right and left body segments was checked with Student's t-test for unpaired data for variables with normal distribution and the Wilcoxon test for variables with non-normal distribution (differences between the control group and canoeists as well as differences between the right and left side in the control group and in the group of canoeists separately).
Spearman's rank correlation coefficient was used to determine the relationship between variables due to the no normal distribution for at least one variable. Statistical power of the test was checked with Cohen's criteria, an effect size � 0.20 and < 0.50 was considered small, � 0.50 and < 0.80 was considered medium and � 0.80 was considered large [9]. The effect size was selected taking into account the group-leg interaction.
Statistical significance was found at p<0.05, while the tendency to significance at p<0.10, p�0.05 was considered statistically insignificant.
All analyses were performed using the Statistica 13.3 software program (TIBCO Software Inc., 2017).

Somatic characteristics and body composition asymmetry
Descriptive statistics of the canoeists and control group are shown in Table 1. Both groups (canoe athletes and control) were homogeneous in terms of body height (t = 1.32, p = 0.194) and body weight (Z = 0.99, p = 0.320). The only statistically significant differences in mean somatic parameters between the two groups were in FM% (t = −2.71, p = 0.010, d = 0.85), which had a large effect size, and FMkg (t = −2.09, p = 0.043, d = 0.65), which had a medium effect size. Canoeists had lower FM% and FMkg compared to controls. Trends toward significance occurred for FFMkg (t = 1.90, p = 0.066, d = 0.58) and PMMkg (t = 1.99, p = 0.054, d = 0.62) with a medium effect size in both cases. Table 2 shows the site-specific body composition. The analysis of FM% side-to-side differentiation indicated significantly higher values of this variable occurred in the left compared to right arm both in canoeists and control as well (p = 0.032 and p<0.001, respectively). Similar situation was observed in the case of legs

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(p = 0.043 and p = 0.001, respectively). When comparing asymmetry level of this parameter between groups, significant difference was noticed in the case of right legs (p = 0.046) and left legs (p = 0.047), but no in arms.
The absolute values for FMkg also showed that left segments were dominant in canoeists and control but the differences were statistically significant only in arms (p = 0.029 and p = 0.017, respectively). Such differences were not observed for the lower extremities. However, the significant differences were noticed between groups in the case of right legs (p = 0.033) and left legs (p = 0.032).
The analysis of FFMkg asymmetry revealed that in the canoeists significantly higher values of this variable occurred in left compared to right arm (p<0.001) and in left compared to right leg (p<0.001). In the control, the significant difference was noticed between left and right leg (p<0.001).
When analyzing PMMkg value it was noticed, that there was significant differentiation in canoeists in the case of arms (p<0.001) and legs (p<0.001), whereas in the control group only in the case of legs (p<0.001).
There were no significant differences between left and right arms as well as legs in FFMkg and PMMkg between the canoeists and control. Table 3 presents the postural stability variables measured in all participants.

Postural stability
There were no significant differences between left and right arms as well as legs in FFMkg and PMMkg between the canoeists and control.
In the case of all stability indices (OSI, APSI and MLSI), the canoeists demonstrated better balance scores than the controls (values closer to zero), though only APSI was significantly different between groups (t = −2.46, p = 0.018, d = 0.82).
When comparing postural indices without separating single-leg positions between the canoeists and controls, the groups were significantly different from each other and there was an interaction between groups in APSI (F = 21.18, p � 0.0001, d = 1.02) and MLSI (F = 12.77, p = 0.001, d = 0.79).

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Statistically significant effects of groups and an interaction of measurement condition were observed for OSI (F = 28.59, p � 0.000). This was influence by a statistically significant difference between the groups in the left leg (p = 0.017, d = 1.16). In the right leg there were no differences between the two groups (p = 0.778).
For APSI, the same observation was made for the right single-leg position (F = 28.84, p = 0.000, d = 1.78). In the control group, the right single-leg position was significantly different than the left single-leg position in OSI (F = 28.59, p = 0.000, d = 1.69) and APSI (F = 28.84, p = 0.000, d = 1.70).

Correlations between somatic characteristics, body composition asymmetry and stability indexes
Taking into consideration basic anthropometric parameters in relation to postural stability it was found, that in canoe athletes, only body height correlated with APSI (r = −0.43); lower body height values correlated with poorer balance scores (Table 4).
Next, side-to-side differences between single-leg stability indices and basic somatic parameters were evaluated. The canoe athlete group demonstrated a statistically significant correlation between body weight and OSI in the right single-leg position (r = −0.49) and left single-leg position (r = −0.45). In both the right and left lower limbs, there was a negative, statistically significant relationship between body weight and OSI such that the lower the body weight, the poorer the OSI result. There was also a negative, statistically significant relationship between body weight and APSI in the left single-leg position (p = 0.043), with athletes with lower body Table 4

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weight achieving poorer APSI results. In the right single-leg positions, a tendency toward significance was observed (p = 0.056). No significant correlations between body weight and single-leg stability indices were observed in the control group. The next question was whether body composition asymmetry determines postural stability in canoeists and control. In the case of canoeists, only in the case of FMkg legs asymmetry correlation appeared with right leg OSI and right leg MLSI postural stability. Among control, FMkg legs asymmetry correlated with OSI whereas FM% legs asymmetry correlated with MLSI. For the remaining bias differences, no correlations were observed with any of the stability indexes.

Discussion
The main objective of the present study was to assess the differentiation among some morphological characteristics with segmental analysis of body composition and postural stability among canoeists and control in order to determine whether athletes are more asymmetric in morphological and functional characteristics of the body than non-athletes. It's seems to be important because previous observations demonstrated that certain somatic characteristics could affect athletes' balance [43,44] and that basic somatic and body composition variables should be taken into account when identifying the most talented canoeists [9,45] and systematically monitored during canoe training programmes [7]. Robinson [46] showed that athletes' anthropometric characteristics have a large impact on competitiveness in the canoe sprint.
In previous studies, elite canoe sprinters in Japan had a height of 171.2±5.50 cm and a body mass of 71.8±8.20 kg, while university canoe athletes (aged 20.6±0.9 years) in Hamano [7] study had a height of 172.8±5.20 cm and a body mass of 70.8±7.80 kg. Young Bulgarian canoe athletes aged 15.12±0.14 years had a height of 172.3±5.51 cm and weight of 66.1±7.29 kg [47]. Some authors have stated that satisfactory results at the international level can only be achieved by canoeists who are 180-190 cm in height [48,49]. The athletes analysed in the present study fulfilled this characteristic, as the average height of the canoe athletes was 181.3 ±6.42 cm, making the athletes taller than the controls (178.9±6.13 cm). The canoe athletes also had higher mean body weight values than the controls (80.1±10.50 kg and 78.1±9.84 kg, respectively). The reported body height and mass of the junior and teenage Polish national canoeing team (aged 18.7 years; 176.9 cm height and 75.5 kg mass) were smaller than those found in our study [8].
Body height and weight are connected to balance. Subjects who are taller and heavier have been found to demonstrate worse balance than their shorter and lighter counterparts [50][51][52]. Hue [52] moreover stated, that body weight alone explained 52% of the variance of static balance in adults, and body weight has been reported to play an important role in balance maintenance in unstable conditions [5]. In the present study, athletes with lower body height demonstrated greater anterior-posterior sway as measured by the APSI. In addition, decreased body weight in the athlete group was connected with OSI in the right and left single-leg position as well as with APSI in the left single-leg position. Relationships between height and weight and postural stability were not observed in the control group.
Athletes had lower levels of body fat and higher levels of fat-free mass and muscle mass than controls, though the differences were small and statistically significant only for fat mass. Hagner-Derengowska [8] showed that canoe athletes had a small fatty tissue percentage and higher fat-free mass values compared to control. This may be attributed to the key goal of training in these athletes being improving strength and speed, which favours muscle tissue hypertrophy [53]. In addition, adipose tissue levels are important in canoeing because they directly affect the submersion of the boat, which in turn impacts the drag on the boat and thus the boat's speed [43]. Moreover, additional weight (especially connected with excess adipose tissue) of the canoe athlete, affects the submersion of the boat, which in turn impacts its drag and thus changes the boat's speed [33].
The canoe athletes in this study were found to have significant asymmetries in fat-free mass and muscle mass between the right and left arms as well as between the right and left legs.
Other authors have stated that long-term canoe paddling does not increase muscle mass asymmetries and that long-term training can actually help to equalise muscle mass in the lower limbs [33]. Also, in the case of fatty tissue (FM%) distribution in arms and legs significant differences were observed both in athletes and control. Kašček Bučinel [44] reported that body symmetry is a critical factor of postural stability. The greater body symmetry connected with less difference between the right and left sides, the better static balance [40]. Particularly among athletes, good postural balance reduces the risk of injuries [54,55] and is an important component of physical performance. On the other hand, higher racing speed of canoeist has been associated with more pronounced asymmetry in lumbar spine flexion in the coronal plane due to prolonged training leading to adaptive changes in the body [24]. However, it remains unclear when asymmetry is acceptable and when it has negative health consequences; thus, further observations are needed.
When compared two groups in morphological asymmetry level it was observed that only in case of lower limb fat tissue distribution significant differentiation appeared.
Previous findings appear to show the opposite tendency regarding the effect of prolonged training on asymmetry, with lower levels of asymmetry in lower limb muscle mass observed in advanced canoe competitors compared to competitors at the beginning of their careers [56]. This may be due to effective general development training, which reduces asymmetries that may have occurred during biological development in young athletes. It is also possible that the training used in young athletes focuses largely on specialised activities, which can cause muscle mass asymmetries. It is well known that in many sports (especially unilateral ones), certain interlimb asymmetry is acceptable, so it should not be automatically associated with worse performance [57,58]. According to Afonso and co-authors [59], some level of asymmetry seems to be treated as the norm rather than something special.
Arol and Eroğlu [29] stated that static and dynamic balance ability is dependent on the specific training methods employed. Standing balance is connected with the ability to adapt and readapt quickly and effectively to a given task and environment [47]. In canoe athletes specifically, balance is a valuable skill.
It should be noted that the canoe athletes in the present study had better balance than the controls. This is in line with the findings yielded by other studies focusing on sports where biomechanical stability is required to maintain balance during activity what is essential for safety and effective work [60]. Stambolieva [47] similarly reported significantly better body postural stability in young male canoe and kayak athletes compared to controls due to the balance requirements specific to these sports, which naturally improve athletes' postural control. Afonso et al. suggest that those with larger asymmetries or lower level of balance, require more attention to improve performance and reduce the risk of overload injuries (bearing in mind that some degree of interlimb asymmetry is not always associated with injury occurrence) [59].

Conclusion
Our findings suggest that the analysed canoeists had statistically lower level of fatty tissue compared to controls. In both groups morphological asymmetry was observed, but in most cases in athletes. For canoeists, side-to-side differentiation was observed for all parameters. The exception was FMkg in the lower extremities, which did not differ significantly. In the case of control symmetry was noticed between legs FMkg, arms FFMkg and arms PPMkg. Statistically significant differences between sides between both groups appeared only in legs FM (%, kg).
The present analysis showed that analysed athletes had better balance than the controls. There were relationships between stature and body weight with postural stability in the case of canoeists. At the different measuring positions, a statistically significant asymmetries were observed in the athletes.
Our work in this study has some limitations like small sample size, lack of dynamic analysis of postural stability, tool used for body composition analysis which should be more accurate, like DXA which is the reference method, lack of the relationship of lateral asymmetry of parameters with the sport level and biomechanical/physical factors or conclusion done for only one gender (men). Nevertheless, we believe that our findings could be a worthwhile consideration about potentially connection between morphological somatic parameters and its side-to-side differentiation with postural stability in canoeists. Future studies should aim to develop the sport-specific level of morphofunctional asymmetry which would be optimal for sport results and health as well.

Practical application
Due to the relationships found in present research between balance and certain somatic parameters with its side-to-side distribution, it is important to systematically monitor body composition and asymmetry among canoe athletes in view of their practical implications during training and in competitive settings. Further studies however are needed, with larger samples of athletes of both sexes and additional age categories to confirm our results.